1. Field of the Invention
The present invention relates to an objective optical system for endoscopes, and more specifically to an objective optical system for endoscopes which comprises a plurality of lens components including one comprising an aperture stop and is configured so as to permit adjusting a distance to an object to be observed by disposing one of optical members having focal lengths different from one another selectively in the vicinity of the aperture stop comprised in the lens component.
2. Description of the Prior Art
In the recent years, there are widely used medical endoscopes which permit observing organs in human bodies with slender portions inserted into human body cavities and elaborately diagnosing abnormal locations by sampling biological tissues with forceps inserted through forceps channels as occasion demands. Further, in industrial fields also, industrial endoscopes are widely used for observing and inspecting interiors of boilors, turbines, engines, chemical plants and so on.
In case of the industrial endoscopes, in particular, a direct viewing type adaptor is used when an object to be observed is located in front of a leading end of the slender inserting portion in a longitudinal direction thereof or a side viewing type adaptor is selected when an inside wall surface or an object to be observed is located in a direction perpendicular to the inserting direction of the inserting portion. For such observation purposes, it is often practised to select an adaptor which has a field angle optimum for the location of the object to be observed simultaneously with the adaptor described above.
Since endoscopes are relatively expensive, it is very effective, from an economical viewpoint, to provide a leading end adaptor type of endoscope which can be equipped with a leading end adaptor having a viewing direction and a field angle optimum for a location of an object to be observed. Though the endoscope which is equipped with such a leading end adaptor permits observing an object in an optimum viewing direction and at an optimum field angle, the endoscope requires any focusing means for observing objects in a variety of observing conditions.
In order to solve the problem described above, the optical system for endoscopes disclosed by Japanese Patent Kokai Publication No. Sho 63-291,019 adopts an objective lens system using a means which moves, along an optical axis, certain lens components disposed in the objective lens system for focusing the optical system on an object to be observed through endoscopes. However, such a focusing means disposed in the optical system to be built in a main body of an endoscope makes it difficult to configure endoscopes compact and is therefore undesirable for use in an optical system for endoscopes which are to be configured compact.
Further, there are known objective optical systems for endoscopes which perform focusing by driving objective lens systems with piezoelectric actuators using piezoelectric elements as well as objective optical systems for endoscopes which permit remote control of objective lens systems with manual controllers utilizing force transmitting means such as wires.
When optical systems for endoscopes are to be focused by moving objective lens systems, however, the optical systems are apt to require tedious operations for focusing. When a wire is used as a force transmitting means, for example, the wire is built in an inserting portion and is moved by operating a mechanism for bending a distal end or bending the inserting portion, thereby making it difficult to keep an optical system for endoscopes in an optimum focused condition and inevitably degrading operability of the endoscope which uses the optical system. Further, the optical system which uses the optical system comprising the wire requires forcusing operations to be repeated each time the endoscope is to be set for observing another object, thereby complicating operations of the endoscope for observations. When the piezoelectric actuator is used in place of the wire for moving an objective lens system, it is difficult to configure a controller for the piezoelectric actuator and the piezoelectric actuator is also undesirable for use in endoscopes which should be configured as compactly as possible.
Furthermore, there is known a leading end adaptor type endoscope, as exemplified by the one disclosed by Japanese Patent Kokai Publication No. Sho 63-298,314, which is configured so as to perform focusing by using a focusing mechanism disposed in a leading end adaptor to be attached to a front lens component built in an objective lens system.
FIG. 2 shows a sectional view illustrating the leading end of the endoscope and the leading end adaptor. As is seen from this drawing, the leading end of the endoscope has a main body M which comprises an observation optical system which consists of an objective lens system O and an image guide fiber bundle IG as well as an illumination light guide fiber bundle LG disposed in parallel with the observation optical system. Further, disposed in the main body M is a wire passage through which a wire WM having an engaging end E extends to a manual controller. Attached to the other end of the wire WM is a rack R which engages with a pinion PI fixed to a control knob arranged on the manual controller.
On the other hand, a light guide fiber bundle LG, an observation optical system which comprises a negative lens component L.sub.1, another lens component L.sub.F attached to a movable frame and a cover glass plate CG are disposed in parallel with each other in the adaptor AD. Attached to the movable frame is the wire WM having the engaging portion E at one end thereof. Further, a tapped ring is rotatably fitted over the adaptor AD.
The main body M of the endoscope is coupled with the adaptor in such a manner that optical axes of the illumination light guide fiber bundles LG disposed therein are aligned with each other and optical axes of sections of the illumination optical systems disposed therein are also aligned with each other, and the adaptor is attached to the main body M of the endoscope by screwing the tapped ring over thread formed on the main body M of the endoscope. In a condition where the adaptor AD is coupled with the main body M of the endoscope as described above, the control knob is turned to move the rack R, thereby moving the movable frame back and forth by way of the wire WM.
The endoscope of the type which is configured so as to perform focusing by moving the lens components disposed in the leading end adaptor type objective optical system as described above has not only a defect that it can hardly have high operability and a compact size, but also another defect that it allows aberrations to be aggravated, a field angle thereof to be varied remarkably and hindrance to be caused for observation when the lens components are moved for a long distance.
Further, the conventional endoscope described above uses an aperture stop having a diameter which is kept fixed for focusing. Therefore, this endoscope provides brightness insufficient for observation of an object located at a long distance when the diameter of the aperture stop is small, for example, to obtain a large depth of field for observing an object located at a short distance. When the diameter of the aperture stop is large for obtaining brightness sufficient for observing an object located at a long distance, in contrast, the endoscope has a depth of field too small for observing an object located at a long distance and is therefore insufficient for practical use. On the other hand, an endoscope which is equipped with an automatic iris requires a large mechanism for controlling an aperture stop and is therefore undesirable as a practical endoscope.